Single-crystal silicon-carbide substrate and polishing solution

ABSTRACT

The present invention relates to a single-crystal silicon-carbide substrate provided with a principal surface having an atomic step-and-terrace structure containing atomic steps and terraces derived from a crystal structure, in which the atomic step-and-terrace structure has a proportion of an average line roughness of a front edge portion of the atomic step to a height of the atomic step being 20% or less.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 15/623,540 filed Jun. 15, 2017, which is in turn adivisional application of U.S. patent application Ser. No. 14/246,556filed Apr. 7, 2014, which is in turn a continuation application ofInternational Application No. PCT/JP2012/075504, filed Oct. 2, 2012,which claims priority to Japanese Patent Application No. 2011-222782,filed on Oct. 7, 2011. The contents of these applications areincorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a single-crystal silicon-carbidesubstrate and a polishing solution, and in more detail, to asingle-crystal silicon-carbide substrate suitable for forming a highquality semiconductor layer by epitaxial growth, and a polishingsolution for obtaining the substrate.

BACKGROUND ART

Because silicon-carbide (SiC) semiconductor has a higher dielectricbreakdown field and a larger saturated drift velocity of electron andthermal conductivity than those of a silicon semiconductor, research anddevelopment are made for realizing a power device capable of operatingin higher speed at a higher temperature than those of the conventionalsilicon device. Above all, an attention has been attracted to thedevelopment of a high-efficient switching element used in a power sourcefor driving a motor of a power-assisted bicycle, an electric vehicle, ahybrid car and the like. To realize such a power device, asingle-crystal silicon-carbide substrate having smooth surface forforming a high quality silicon-carbide semiconductor layer by epitaxialgrowth is necessary.

Furthermore, a blue laser diode has attracted an attention as a lightsource for recording information in high density, and additionally,needs to a white diode as a light source in place of a fluorescent lampor an electric bulb are increasing. Such a light-emitting element isprepared using a gallium nitride (GaN) semiconductor, and asingle-crystal silicon-carbide substrate is used as a substrate forforming a high quality gallium nitride semiconductor layer.

High processing accuracy is required in flatness of a substrate,smoothness of a substrate surface and the like to the single-crystalsilicon-carbide substrate used in such use applications. Furthermore,high cleanability is required regarding a residue such as an abrasive orthe like derived from a polishing agent. However, because asilicon-carbide single crystal has extremely-high hardness and excellentcorrosion resistance, workability in preparing a substrate is poor, andit is difficult to obtain a single-crystal silicon-carbide substratehaving high smoothness while maintaining high polishing rate.Furthermore, even in the removal of the abrasive, because thesilicon-carbide single crystal has excellent corrosion resistance, amethod of removing an abrasive residue by lift-off using a chemical suchas hydrofluoric acid as used in the cleaning of a silicon substrate isdifficult to be applied. Therefore, it is difficult to obtain asubstrate surface having high cleanliness.

In general, a smooth surface of a single crystal semiconductor substrateis formed by polishing. In the case of polishing silicon-carbide singlecrystal, the surface thereof is mechanically polished using an abrasivesuch as diamond or the like that is harder than silicon carbide as anabrasive material to form a smooth surface. In such a case, finescratches according to a particle size of the diamond abrasive areincorporated in the surface of the single-crystal silicon-carbidesubstrate polished with the diamond abrasive. Furthermore, because anaffected layer having mechanical strain is generated on the surface, thesmoothness of the surface of the substrate is not sufficient as is.

In the production of a single crystal semiconductor substrate, chemicalmechanical polishing (hereinafter referred to as “CMP”) technique hasbeen used as a method for smoothening the surface of a semiconductorsubstrate after mechanically polishing. CMP is a method of convertingthe surface of a material to be processed to an oxide or the like byutilizing a chemical reaction such as oxidation and removing the oxideformed using an abrasive having hardness lower than that of the materialto be processed, thereby polishing the surface. This method has theadvantage that an atomic step-and-terrace structure comprising atomicsteps and terraces derived from a crystal structure is formed withoutgenerating strain on the surface of a material to be process, andextremely-smooth surface in atomic level can be formed.

The formation of a silicon-carbide semiconductor layer on asingle-crystal silicon-carbide substrate by epitaxial growth isperformed by depositing silicon atoms and carbon atoms by a thermal CVDmethod on an extremely-smooth surface in atomic level on which an atomicstep-and-terrace structure has been formed by the CMP. In such a case,the front edge of the atomic step becomes the origin of epitaxialgrowth. Therefore, to obtain a high quality silicon-carbidesemiconductor layer free of crystal defect, as surface propertiesrequired in a single-crystal silicon-carbide substrate, not only anatomic step-and-terrace structure derived from a crystal structure isformed, but high processing accuracy is required in the shape of theatomic step formed. Particularly, it is necessary in the front edgeportion of the atomic step that crystal defect derived from mechanicaldamage by polishing is suppressed.

In the present description, the “atomic step-and-terrace structure”means a micro step-like structure comprising a plurality of flat“terraces” provided so as to be parallel to each other through stepdifference along a principal surface of a single crystal substrate and“atomic steps” that are step difference parts connecting the terraces. Alinear site at which the upper edge of the atomic step contacts theterrace is defined as a “front edge portion of an atomic step”. The“terrace”, “atomic step” and “front edge portion of an atomic step” arefurther described hereinafter.

To form a high quality silicon-carbide semiconductor layer, there is aproposed method of conducting CMP by a colloidal silica slurry or acolloidal silica slurry containing an oxidizing agent after diamondpolishing to form a high smoothness surface having an atomicstep-and-terrace structure derived from the crystal structure, andfurther conducting etching by a gas-phase method (e.g., see PatentDocument 1). In Patent Document 1, in the case where a silicon-carbidesemiconductor layer is film-formed without conducting the etchingtreatment, step bunching occurs by an oxide formed extremely-near thesurface of a substrate after CMP, however, by conducting the etchingtreatment, only a surface oxide layer generated by CMP can be removedwhile maintaining high smoothness of the surface of a substrate afterCMP and crystal defect such as step bunching can be suppressed.

However, in Patent Document 1, although the formation of an atomicstep-and-terrace structure derived from the crystal structure isconsidered, the influence of the edge shape of an atomic step and thecrystal defect, to epitaxial growth of a crystal is not considered atall. Furthermore, merely suppressing the crystal defect of asilicon-carbide semiconductor layer by etching is not sufficient toobtain a high quality semiconductor layer. Further, higher polishingrate is required to be realized from the standpoint of cost.

A polishing composition containing a silica adhesive, an oxidizing agent(oxygen donor) such as hydrogen peroxide and vanadate is conventionallyproposed as a polishing agent for polishing the surface of asingle-crystal silicon-carbide substrate in high polishing rate andsmoothly (e.g., see Patent Document 2).

However, in the polishing composition of Patent Document 2, there wasthe problem that a polishing rate to a single-crystal silicon-carbidesubstrate is low and time required for polishing is very long. There wasfurther problem that although an atomic step-and-terrace structure isformed on the surface after polishing, the front edge portion of anatomic step becomes a shape having crack and dent due to mechanicaldamage at polishing and crystal defect occurs. Furthermore, there was aproblem that silica abrasives that could not be removed by cleaningremain on a substrate and the abrasive residue causes crystal defect ofa semiconductor layer epitaxially grown on the surface of a substrateafter polishing.

CITATION LIST Patent Literature

Patent Document 1: WO2010-090024

Patent Document 2: JP-A-2008-179655

SUMMARY OF THE INVENTION Problems that the Invention is to Solve

The present invention has been made to solve the above problems, and hasan object to provide a single-crystal silicon-carbide substrate suitablefor epitaxially growing a high quality semiconductor layer free ofcrystal defect and a polishing solution for obtaining the single-crystalsilicon-carbide substrate by CMP.

Means for Solving the Problem

The single-crystal silicon-carbide substrate according to the presentinvention has a principal surface having an atomic step-and-terracestructure comprising atomic steps and terraces derived from a crystalstructure, wherein the atomic step-and-terrace structure has aproportion of an average line roughness of a front edge portion of theatomic step to a height of the atomic step being 20% or less.

In the single-crystal silicon-carbide substrate according to the presentinvention, the principal surface is preferably a surface on which acrystal is to be epitaxially grown to form a silicon-carbidesemiconductor layer or a gallium-nitride semiconductor layer.

The polishing solution according to the present invention is a polishingsolution for chemically and mechanically polishing a principal surfaceof a predetermined surface direction of a single-crystal silicon-carbidesubstrate, such that the principal surface has an atomicstep-and-terrace structure comprising atomic steps and terraces derivedfrom a crystal structure, and that the atomic step-and-terrace structurehas a proportion of an average line roughness of a front edge portion ofthe atomic step to a height of the atomic step being 20% or less, inwhich the polishing solution comprises an oxidizing agent containing atransition metal having oxidation-reduction potential of 0.5V or more,and water, and does not contain an abrasive.

In the polishing solution according to the present invention, theoxidizing agent is preferably permanganate ions. And the permanganateions is contained in an amount of 0.05% by mass or more and 5% by massor less based on the total amount of a polishing agent. Further, thepolishing solution preferably has pH of 11 or less and more preferably 5or less.

Effect of the Invention

The single-crystal silicon-carbide substrate of the present inventionhas an atomic step-and-terrace structure derived from the crystalstructure, wherein the proportion of an average line roughness (R) of afront edge portion of the atomic step to a height (h) of the atomic stepis 20% or less. Because crystal defect or the like is suppressed on thefront edge portion which is the origin of epitaxial crystal growth in astep flow method, by epitaxially growing on a principal surface of thesingle-crystal silicon-carbide substrate, a high quality silicon-carbidesemiconductor layer or gallium-nitride semiconductor layer can beobtained.

The polishing solution of the present invention contains an oxidizingagent containing a transition metal having oxidation-reduction potentialof 0.5V or more, and water, and does not contain an abrasive. Therefore,when a principal surface in a predetermined surface direction of thesingle-crystal silicon-carbide substrate is chemically and mechanicallypolished by using the polishing solution, high smoothness surface havingan atomic step-and-terrace structure derived from the crystal structureand free of crystal defect in the front edge portion of the atomic stepdue to mechanical damage in polishing can be obtained. Furthermore, thepolishing solution does not generate an abrasive residue on thesingle-crystal silicon-carbide substrate after cleaning.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B schematically show atomic step-and-terrace structureformed on a principal surface in the single-crystal silicon-carbidesubstrate of an embodiment of the present invention; FIG. 1A is a planeview and FIG. 1B is an enlarged perspective view.

FIG. 2 is a view showing a crystal structure of 4H-SiC single crystal.

FIG. 3 is a view showing an example of a polishing apparatus usable inthe polishing using the polishing solution of an embodiment of thepresent invention.

FIGS. 4A and 4B schematically show an atomic step-and-terrace structureformed on a principal surface in the single-crystal silicon-carbidesubstrate polished by using the conventional polishing agent solution;FIG. 4A is a plane view and FIG. 4B is an enlarged perspective view.

FIG. 5 is a view schematically showing epitaxial growth by a step flowmethod on a single-crystal silicon-carbide substrate.

FIG. 6 is a view showing measurement position of line roughness of afront edge portion of the atomic step-and-terrace structure formed afterCMP polishing in Examples 1 to 6.

MODE FOR CARRYING OUT THE INVENTION

The embodiment of the present invention is described below.

<Single-Crystal Silicon-Carbide Substrate>

The single-crystal silicon-carbide substrate of an embodiment of thepresent invention has a high smoothness principal surface having anatomic step-and-terrace structure in which a flat terrace 1 region andan atomic step 2 of a step difference region, which are derived from acrystal structure, are alternately continued, as schematically shown inFIG. 1A and FIG. 1B. In the atomic step-and-terrace structure, a frontedge portion 2 a at which the upper edge of the atomic step 2 contactsthe terrace 1 shows a straight line state, and is free of curvature,crack and dent. The width of the terrace 1 is nearly the same in all ofterraces, and is nearly uniform in each terrace. C axis shown in FIG. 1Bis a vertical direction to the paper face in FIG. 1A.

In the atomic step-and-terrace structure, the proportion of an averageline roughness (R) of the front edge portion 2 a of the atomic step 2 toa height (h) of the atomic step 2 is 20% or less. That is, (R/h)×100≦20.The R/h can be considered as an index showing the degree of mechanicaldamage to the front edge portion 2 a in the atomic step-and-terracestructure.

The principal surface having the atomic step-and-terrace structure is aprincipal surface in a predetermined surface direction and is aprincipal surface at a predetermined off angle to the C axis. Theaverage line roughness (R) of the front edge portion 2 a is arithmeticalmean roughness (Ra) of the center line of a cross-section roughnesscurve of the front edge portion 2 a, and can be measured by, forexample, the following method. That is, a predetermined range (e.g., arange of 2 μm horizontal×1 μm vertical) on a principal surface of thesingle-crystal silicon-carbide substrate is observed with AFM (atomicforce microscope), the respective arithmetical mean roughnesses (Ra) ofa plurality of the front edge portions 2 a fallen within the above rangeare measured from the obtained AFM image, and R is obtained as theiraverage value.

The height (h) of the atomic step is about 0.25 nm in the single-crystalsilicon-carbide substrate.

For example, a 4H-SiC substrate has the crystal structure shown in FIG.2, and is that 1/4 of C0 (1.008 nm) that is a crystal lattice interval(lattice constant) in a C axis direction is the height (h) of the atomicstep. That is, in the 4H-SiC substrate, the height (h) of the atomicstep is a value (about 0.25 nm) calculated from 1.008 nm/4.

The height (h) of the atomic step in a 6H-SiC substrate is about 0.25 nmsimilar to the 4H-SiC substrate. That is, in the 6H-SiC substrate, thelattice constant C0 in a C axis direction is 1.542 nm, and 1/6 of thisvalue is the height (h) of the atomic step. Therefore, the height (h) ofthe atomic step is about 0.25 nm.

The 4H-SiC substrate and 6H-SiC substrate are described in the item ofan object to be polished.

In the single-crystal silicon-carbide substrate of the embodiment, theprincipal surface in a predetermined surface direction has an atomicstep-and-terrace structure derived from the crystal structure and has ahigh smoothness, and in addition, the proportion of the average lineroughness (R) of the front edge portion 2 a to the height (h) of theatomic step 2 is 20% or less. As a result, crystal defect or the like issuppressed in the front edge portion 2 a which is the origin ofepitaxial crystal growth in a step flow method. Therefore, a highquality silicon-carbide semiconductor layer or gallium nitridesemiconductor layer can be obtained by epitaxially growing crystals onthe principal surface of this single-crystal silicon-carbide substrate.The step flow method is described in detail in the item of epitaxialgrowth described below.

Where the proportion of the average line roughness (R) of the front edgeportion 2 a to the height (h) of the atomic step 2 exceeds 20%, crystaldefect and the like due to mechanical damage of the front edge portion 1a in the atomic step-and-terrace structure become large, and thus a highquality silicon-carbide semiconductor layer or gallium-nitrilesemiconductor layer cannot be formed by epitaxial growth.

Such a principal surface of the single-crystal silicon-carbide substratehaving excellent shape at the front edge portion 2 a and in whichcrystal defect at the portion is suppressed can be obtained byconducting CMP by using the polishing solution of the present inventionthat contains an oxidizing agent having large oxidation power andcontaining a transition metal having oxidation-reduction potential of0.5V or more and does not substantially contain an abrasive.

<Polishing Solution>

The polishing solution of an embodiment of the present invention is apolishing solution for chemically and mechanically polishing a principalsurface in a predetermined surface direction of a single-crystalsilicon-carbide substrate, and the polishing solution contains anoxidizing agent containing a transition metal having oxidation-reductionpotential of 0.5V or more and water, and does not contain an abrasive.

By conducting CMP of a principal surface of a single-crystalsilicon-carbide substrate by using this polishing solution, scratch tothe surface and crystal defect of the front edge portion of the atomicstep, due to mechanical damage in polishing can be suppressed. Then, asdescribed before, the polished principal surface having the atomicstep-and-terrace structure derived from crystal structure and in whichthe proportion of the average line roughness (R) of the front edgeportion to the height (h) of the atomic step is 20% or less((R/h)×100≦20) (hereinafter referred to as a “polished principalsurface”) can be obtained.

Furthermore, a substrate having high hardness and high chemicalstability, such as a single-crystal silicon-carbide substrate, can bepolished in sufficiently-high polishing rate by using such an oxidizingagent having strong oxidation power in the atomic level processing. Inthe case where this polishing solution has been used, an abrasive doesnot remain on the single-crystal silicon-carbide substrate aftercleaning. As a result, the occurrence of crystal defect due to anabrasive residue can be prevented.

As the oxidizing agent contained in the polishing solution of theembodiment of the present invention, permanganate ion is preferred, andits content is preferably 0.05% by mass or more and 5% by mass or less.The pH of the polishing solution is preferably 11 or less, and morepreferably 5 or less. To adjust the pH toll or less, a pH adjuster canbe added to the polishing solution. In the case where the pH of thepolishing solution is 11 or less, the oxidizing agent acts effectively,and as a result, polishing rate is high and polishing performances aregood.

Each component and pH of the polishing solution of the present inventionis described below.

(Oxidizing Agent)

The oxidizing agent containing a transition metal havingoxidation-reduction potential of 0.5V or more contained in the polishingsolution of the embodiment of the present invention forms an oxide layeron a face to be polished of the single-crystal silicon-carbide substratethat is an object to be polished. The polishing of the object to bepolished is accelerated by removing the oxide layer by mechanical powerfrom the face to be polished. That is, although the silicon carbidesingle crystal that is a non-oxide is a polishing-difficult material, anoxide layer can be formed on the surface by an oxidizing agentcontaining a transition metal having oxidation-reduction potential of0.5V or more in the polishing solution. The oxide layer formed has lowhardness and is easy to be polished as compared with that of the objectto be polished, and thus the oxide layer can be removed by the contactwith a polishing pad that does not contain an abrasive therein.Therefore, sufficiently-high polishing rate can be achieved. Anoxidation-reduction potentiometer generally commercially available canbe used for an oxidation-reduction potential measurement method. Asilver/silver chloride electrode in which a saturated potassium chlorideaqueous solution was used as an internal liquid can be used as areference electrode, and a metal electrode such as platinum can be usedas a working electrode. Regarding a temperature and pH of the aqueoussolution at measuring, the measurement is performed at room temperaturenear 25° C., and the pH can be measured by preparing samplesappropriately adjusted.

In the present invention, the “face to be polished” is a face of anobject to be polished, which is polished, and means, for example, asurface.

Examples of the oxidizing agent containing a transition metal havingoxidation-reduction potential of 0.5V or more and contained in thepolishing solution include permanganate ion, vanadate ion, dichromateion, cerium ammonium nitrate, iron (III) nitrate nonahydrate, silvernitrate, phosphotungstic acid, silicotungstic acid, phosphomolybdicacid, phosphotungstomolybdic acid, and phosphovanadomolybdic acid.Permanganate ion is particularly preferred. Permanganate such aspotassium permanganate or sodium permanganate is preferred as a supplysource of the permanganate ion.

The reasons that permanganate ion is particularly preferred as theoxidizing agent in the polishing of the single-crystal silicon-carbidesubstrate are described below.

(1) Permanganate ion has strong oxidation power that oxidizessilicon-carbide single crystal.

In the case where the oxidation powder of the oxidizing agent is tooweak, the reaction with a face to be polished of the single-crystalsilicon-carbide substrate becomes insufficient, and as a result,sufficiently-smooth surface cannot be obtained. Oxidation-reductionpotential is used as an index of oxidation power that an oxidizing agentoxidizes a material. Oxidation-reduction potential of permanganate ionis 1.70V, and the oxidation-reduction potential is higher than that ofpotassium perchlorate (KClO₄) (oxidation-reduction potential: 1.20V) andsodium hypochlorite (NaClO) (oxidation-reduction potential: 1.63V) thatare generally used as an oxidizing agent.

(2) Permanganate ion has large reaction rate.

The permanganate ion has a large reaction rate of an oxidation reactionas compared with hydrogen peroxide (oxidation-reduction potential:1.76V) that is known as an oxidizing agent having strong oxidationpower, and therefore can quickly exhibit the strength of oxidationpower.

(3) Permanganate ion has low toxicity to human body and is safe.(4) Permanganate is completely dissolved in water that is a dispersionmedium described below under the condition of dissolved concentration orlower concentration that is obtained from a solubility curve and dependson water temperature. Therefore, the dissolved residue does notadversely affect smoothness of a substrate.

To obtain a high polishing rate, the content (concentration) of thepermanganate ion in the polishing solution is preferably from 0.05% bymass to 5% by mass. In the case of less than 0.05% by mass, the effectas an oxidizing agent is not expected, and there is a concern that verylong time is required to form a smooth surface by polishing or scratchoccurs on a face to be polished. In the case where the content ofpermanganate ion exceeds 5% by mass, depending on a temperature, thepermanganate is not completely dissolved and precipitates, and there isa concern that scratch is generated by that solid permanganate contactsa face to be polished. The content of the permanganate ion contained inthe polishing solution is more preferably 0.1% by mass or more and 4% bymass or less, and particularly preferably 0.2% by mass or more and 3.5%by mass or less.

(Abrasive)

The polishing solution of an embodiment of the present invention ischaracterized by substantially not containing a polishing abrasive suchas silicon oxide (silica) particles, cerium oxide (ceria) particles,aluminum oxide (alumina) particles, zirconium oxide (zirconia)particles, and titanium oxide (titania) particles. Because the polishingsolution does not contain an abrasive and abrasive residue is notgenerated after cleaning as described before, the occurrence of crystaldefect due to abrasive residue can be prevented. Furthermore, there areadvantages that the polishing solution can be used without payingattention to dispersibility of an abrasive, and because aggregation ofan abrasive does not substantially occur, mechanical damage to a face tobe polished is suppressed.

(pH and pH adjuster)

The pH of the polishing solution of an embodiment of the presentinvention is preferably 11 or less, more preferably 5 or less, andparticularly preferably 3 or less, from the standpoint of polishingperformances. In the case where the pH exceeds 11, there is a concernthat not only sufficient polishing rate is not obtained, but smoothnessof a face to be polished is deteriorated.

The pH of the polishing solution can be adjusted by theaddition/blending of an acidic or basic compound that is a pH adjuster.Examples of the acid that can be used include inorganic acids such asnitric acid, sulfuric acid, phosphoric acid, and hydrochloric acid;saturated carboxylic acids such as formic acid, acetic acid, propionicacid, and butyric acid; hydroxy acids such as lactic acid, malic acidand citric acid; aromatic carboxylic acids such as phthalic acid andsalicylic acid; dicarboxylic acids such as oxalic acid, malonic acid,succinic acid, glutaric acid, adipic acid, fumaric acid, and maleicacid; and organic acids such as amino acid and heterocyclic carboxylicacids. Nitric acid and phosphoric acid are preferably used, and ofthose, nitric acid is particularly preferably used. Examples of thebasic compound that can be used include ammonia, lithium hydroxide,potassium hydroxide, sodium hydroxide, quaternary ammonium compound suchas tetramethyl ammonium, and organic amines such as monoethanolamine,ethylethanolamine, diethanolamine, and propylenediamine. Potassiumhydroxide and sodium hydroxide are preferably used, and of those,potassium hydroxide is particularly preferably used.

The content (concentration) of those acidic or basic compounds is anamount that adjusts the pH of the polishing solution to a predeterminedrange (pH 11 or less, more preferably 5 or less, and still morepreferably 3 or less).

(Water)

In the polishing solution of an embodiment of the present invention,water is contained as a dispersion medium. Water is a medium fordispersing and dissolving the oxidizing agent described above andoptional components described below that are added as necessary. Thewater is not particularly limited, but pure water, ultrapure water andion-exchanged water (deionized water) are preferred from the standpointsof influence to blending components, contamination of impurities andinfluence to pH.

(Preparation of Polishing Solution and Optional Components)

The polishing solution of an embodiment of the present invention isprepared so that the respective components described above are containedin the predetermined proportions and are mixed so as to become uniformlydissolved and mixed state, and then used. For mixing, use can be made ofa stirring mixing method by stirring blades which is generally used inthe production of a polishing solution. It is not necessary to supplythe polishing solution to the polishing site as one in which all ofpolishing components as constituents have been previously mixed.Polishing components may be mixed when supplying to the polishing side,thereby forming a composition of a polishing solution.

The polishing solution of an embodiment of the present invention canappropriately contain lubricants, chelating agents, reducing agents,thickeners, viscosity adjusters, corrosion inhibitors and the like, asnecessary as long as it does not conflict with the spirit of the presentinvention. However, in the case where those additives have the functionof an oxidizing agent or an acid or basic compound, those additives arehandled as an oxidizing agent or an acid or basic compound.

As the lubricant, use can be made of anionic, cationic, nonionic, andamphoteric surfactants, polysaccharides, water-soluble polymers and thelike. As the surfactant, use can be made of those having a hydrophobicgroup such as an aliphatic hydrocarbon group or an aromatic hydrocarbongroup, or which has at least one of bonding groups such as ester, etherand amide and linking groups such as acyl group and alkoxyl groupintroduced in these hydrophobic group, and those having a hydrophilicgroup having carboxylic acid, sulfonic acid, sulfuric acid ester,phosphoric acid, phosphoric acid ester and amino acid. As thepolysaccharide, use can be made of alginic acid, pectin, carboxymethylcellulose, curdlan, pullulan, xanthan gum, carrageenan, gellan gum,locust bean gum, gum arabic, tamarind, and psyllium. As thewater-soluble polymer, use can be made of polyacrylic acid, polyvinylalcohol, polyvinyl pyrrolidone, polymethacrylic acid, polyacrylamide,polyaspartic acid, polyglutamic acid, polyethylene imine,polyacrylamine, and polystyrene sulfonate.

<Polishing Method>

To conduct the polishing by using the polishing solution of anembodiment of the present invention, the conventional polishing pad thatdoes not contain an abrasive therein is used, the polishing pad iscontacted with a face to be polished of a single-crystal silicon-carbidesubstrate that is an object to be polished, while supplying thepolishing solution to the polishing pad, and the polishing is conductedby relative movement of those. The object to be polished is describedbelow.

In this polishing method, the conventional polishing apparatus can beused as a polishing apparatus. An example of the usable polishingapparatus is shown in FIG. 3.

In the polishing apparatus 10 shown in FIG. 3, a polishing surface plate11 is provided in the state of being rotatably supported around itsvertical shaft center C1. The polishing surface plate 11 isrotation-driven in a direction shown by an arrow in the Figure by asurface plate drive motor 12. A conventional polishing pad 13 that doesnot contain an abrasive therein is adhered to the upper surface of thepolishing surface plate 11.

On the other hand, at an eccentric position from the shaft center C1 onthe polishing surface plate 11, a substrate holding member (carrier) 15that holds an object 14 to be polished on the lower surface thereof byadsorption or using a holding frame is supported rotatably around itsshaft center C2 and movably in a direction of the shaft center C2. Thesubstrate holding member 15 is constituted so as to rotate in adirection shown by an arrow by a carrier drive motor not shown or arotation moment received from the polishing surface plate 11. Asingle-crystal silicon-carbide substrate that is the object 14 to bepolished is held on the lower surface of the substrate holding member15, that is, a surface facing the polishing pad 13. The object 14 to bepolished is pushed to the polishing pad 13 by a predetermined load.

A dropping nozzle 16 or a spray nozzle (not shown) is provided in thevicinity of the substrate holding member 15, and the above-describedpolishing solution 17 send from a tank not shown is supplied on thepolishing surface plate 11.

In polishing by the polishing apparatus 10, the object 14 to be polishedheld on the substrate holding member 15 is pushed to the polishing pad13 in the state that the polishing surface plate 11 and the polishingpad 13 adhered thereto, and the substrate holding member 15 and theobject 14 to be polished held on the lower surface thereof arerotation-driven around the respective axis centers by the surface platedrive motor 12 and the carrier drive motor, while supplying thepolishing solution 17 to the surface of the polishing pad 13 from thedropping nozzle 16 or the like. By this operation, the face to bepolished of the object 14 to be polished, that is, the surface facingthe polishing pad 13, is chemically and mechanically polished.

The substrate holding member 15 may perform not only rotational motionbut straight-line motion. Further, the polishing surface plate 11 andthe polishing pad 13 may not perform rotational motion and for examplemay move in one direction by a belt system.

As the polishing pad 13, use can be made of the conventional polishingpad comprising a non-woven fabric or a porous resin such as foamedpolyurethane and not containing an abrasive. To accelerate the supply ofthe polishing solution 17 to the polishing pad 13 or to keep a constantamount of the polishing solution 17 in the polishing pad 13, grooveprocessing such as a lattice pattern, a concentric pattern or a helicalpattern may be applied to the surface of the polishing pad 13.Furthermore, if necessary, the polishing may be performed whileperforming conditioning of the surface of the polishing pad 13 bycontacting a pad conditioner with the surface of the polishing pad 13.

Polishing conditions by the polishing apparatus 10 are not particularlylimited. Polishing pressure can be further increased by pushing thepolishing pad 13 to the substrate holding member 15 by applying load,thereby improving the polishing rate. The polishing pressure ispreferably from about 5 to 80 kPa, and is more preferably from about 10to 50 kPa from the standpoints of uniformity of polishing rate, flatnessand prevention of polishing defect such as scratch in a face to bepolished. The number of revolution of the polishing surface plate 11 andthe substrate holding member 15 is preferably from about 50 to 500 rpm,but is not limited to this. The amount of the polishing solution 17supplied is appropriately adjusted and selected by the composition ofthe polishing solution and the polishing conditions described above.

<Object to be Polished>

The object to be polished that is polished by using the polishingsolution of an embodiment of the present invention is a single-crystalsilicon-carbide substrate or a single-crystal gallium-nitride substratethat is a non-oxide single crystal, and is more preferably asingle-crystal silicon-carbide substrate. More specifically, asingle-crystal silicon-carbide substrate having crystal structure of3C-SiC, 4H-SiC or 6H-SiC can be mentioned. The 3C-, 4H- and 6H- showcrystal polymorph of silicon carbide determined by the lamination orderof Si—C pair. High polishing rate can be achieved by using the polishingsolution of the embodiment. Furthermore, a principal surface (polishedprincipal surface) having the following surface properties can beobtained.

In each case of using the polishing solution of an embodiment of thepresent invention and the case of using a conventional polishing slurrycontaining hydrogen peroxide as an oxidizing agent and colloidal silicaabrasive, respective surface properties of the principal surface of thesingle-crystal silicon-carbide substrate obtained by CMP and theformation of a semiconductor layer by epitaxial growth onto the polishedprincipal surface are described below by referring to the drawings.

<Surface Properties of Polished Principal Surface>

Mechanical damage due to diamond polishing is generated on the principalsurface of the single-crystal silicon-carbide substrate after diamondpolishing that is a pre-step of a CMP step, and not only scratch isformed on the surface, but an affected layer in which defect such ascrystal strain was generated inside is formed. By conducting polishingfor a considerably long period of time, even CMP by the conventionalpolishing slurry can remove the affected layer generated due to diamondpolishing, and the polished principal surface having an atomicstep-and-terrace structure in which a flat terrace 1 region and anatomic step 2 of a step difference region, which are derived from acrystal structure, are alternately continued as schematically shown inFIGS. 4(a) and 4(b), can be obtained. The C axis shown in FIG. 4B is adirection vertical to the paper face in FIG. 4A.

However, in CMP by the conventional polishing slurry described above,the front edge portion 2 a of the atomic step 2 becomes a shape havingcrack or dent due to mechanical damage at polishing, and crystal defectoccurs. Excessive polishing of the front edge portion 2 a by an abrasivehaving strong mechanical action is considered as the cause of thecrystal defect.

On the other hand, the polishing solution of an embodiment of thepresent invention does not substantially contain abrasive. Therefore, inthe atomic step-and-terrace structure formed on the polished principalsurface, mechanical damage applied to the front edge portion 2 a of theatomic step is considerably reduced. As a result, the front edge portion2 a free of crack, dent and crystal defect can be formed as shown inFIG. 1A and FIG. 1B, and processing accuracy in atomic level in whichsmoothness is high and the front edge portion 2 a maintains excellentshape can be achieved. Further, in the case of using the polishingsolution of the embodiment, by the action of the contained oxidizingagent having strong oxidation power, an affected layer due to diamondpolishing can be promptly removed in high polishing rate by onlymechanical action by a polishing pad having hardness lower than that ofan abrasive, even though mechanical action by an abrasive is notapplied. Accordingly, high processing accuracy in atomic level in whichdamage to the face to be polished of a single-crystal silicon-carbidesubstrate has been suppressed is possible.

<Epitaxial Growth>

Mechanism of epitaxial growth of a semiconductor layer on asingle-crystal silicon-carbide substrate by a step flow method, and arole of a front edge portion of an atomic step are described on thebasis of FIG. 5.

To epitaxially grow, for example, a silicon carbide semiconductor layeron a single-crystal silicon-carbide substrate, silicon atoms and carbonatoms are deposited by a thermal CVD method on the polished principalsurface having formed thereon an atomic step-and-terrace structure ofthe single-crystal silicon-carbide substrate, followed by crystalgrowth. Each atom adhered to the terrace 1 of the atomicstep-and-terrace structure reaches the front edge portion 2 a and bondsto an atom having dangling bond of the front edge portion 2 a, andcrystal is grown in a horizontal direction (a direction perpendicular tothe front edge portion 2 a; shown by an arrow in FIG. 5) on the surfaceof the terrace 1, thereby a film is formed. That is, the front edgeportion 2 a of the atomic step 2 functions as the origin of crystalgrowth in epitaxial growth.

It is known that crystal quality of a semiconductor layer film-formed ona single-crystal silicon-carbide substrate is strongly influenced bycrystal defect and surface state of the substrate. Examples of thecrystal defect of the single-crystal silicon-carbide substrate includemicropipe defect, screw dislocation defect, and edge dislocation defect.Examples of the surface state include scratch by polishing, adhesion ofcontamination such as an abrasive derived from a polishing agent to thesurface of the terrace 1, and a surface oxide of the terrace 1.Regarding the front edge portion 2 a of the atomic step 2 that is theorigin of epitaxial growth, the propagation of defect present on thisportion is considered. Therefore, to film-form higher qualitysemiconductor layer, not the polishing in which only the formation of anatomic step-and-terrace structure has been considered, but processingaccuracy in atomic level in which the shape of the front edge portion 2a of the atomic step 2 and the crystal defect have been considered isnecessary.

Considering from the above point, the polished principal surface of thesingle-crystal silicon-carbide substrate obtained by using theconventional slurry is that because the atomic step-and-terracestructure is formed and smoothened, crystal defect of a semiconductorlayer due to scratch and the like on the surface of the terrace 1 can besuppressed, but because the front edge portion 2 a that is the origin ofcrystal growth has crack, dent and the like and becomes the shape whichthe crystal defect is unavoidable, high quality silicon-carbidesemiconductor layer or gallium-nitride semiconductor layer can not beformed thereon by epitaxial growth.

On the other hand, in CMP processing using the polishing solution of anembodiment of the present invention, a high-smooth principal surfacehaving an atomic step-and-terrace structure in which mechanical damageto the front edge portion 2 a of the atomic step 2 has been suppressedin the atomic step-and-terrace structure can be obtained, and as aresult, crystal growth of higher quality semiconductor layer becomespossible. Furthermore, because the polishing solution of an embodimentof the present invention does not contain an abrasive, an abrasive doesnot remain on the surface of a single-crystal silicon-carbide substrateeven after cleaning, and crystal defect of a semiconductor layer due toabrasive residue derived from a polishing agent can be prevented.

EXAMPLES

The present invention is specifically described below by reference toexamples and comparative examples, but the invention is not limited tothose examples. Examples 1 to 4 are examples of the present invention,and Examples 5 and 6 are comparative examples.

(1) Preparation of Polishing Solution and Polishing Agent Solution

(1-1)

Each polishing solution of Examples 1 to 4 was prepared as follows. Purewater was added to potassium permanganate that is an oxidizing agentshown in Table 1, followed by stirring for 10 minutes using stirringblades. As a pH adjuster, nitric acid in Examples 1 to 3 and potassiumhydroxide in Example 4 were gradually added to the resulting solutionwhile stirring to adjust to a predetermined pH shown in Table 1. Thus,each polishing solution was obtained. The content (concentration: % bymass) of potassium permanganate that is an oxidizing agent used in eachexample to the whole polishing solution is shown in Table 1. Theconcentration of an oxidizing agent in Table 1 is not a concentration ofpermanganate ions, but is a concentration of potassium permanganate.

(1-2)

Polishing agent solutions of Examples 5 and 6 were prepared as follows.In Example 5, pure water was added to a colloidal silica dispersionhaving a primary particle size of 40 nm, a secondary particle size ofabout 70 nm and a silica solid content of about 40 wt %, followed bystirring for 10 minutes by using stirring blades. Ammonium vanadate as ametal salt was added to the resulting solution while stirring, andhydrogen peroxide was added last thereto, followed by stirring for 30minutes. Thus, a polishing agent solution adjusted to each componentconcentration shown in Table 1 was obtained.

In Example 6, pure water was added to a colloidal silica dispersionhaving a primary particle size of 80 nm, a secondary particle size ofabout 110 nm and a silica solid content of about 40 wt %, followed bystirring for 10 minutes. Potassium permanganate as an oxidizing agentwas added to the resulting solution while stirring, and nitric acid wasthen gradually added thereto to adjust to pH as shown in Table 1. Thus,a polishing agent solution was obtained. The content (concentration: %by mass) of each component used in Examples 5 and 6 to the wholepolishing agent is shown in Table 1.

The concentration of the oxidizing agent in Table 1 is not aconcentration of permanganate ions, but is a concentration of potassiumpermanganate. The primary particle size of silica particles blended inExamples 5 and 6 was obtained by converting from a specific surface areaobtained by a BET method, and the secondary particle size was measuredby using Microtrac UPA (manufactured by Nikkiso Co., Ltd) that is adynamic light-scattering particle size analyzer.

(2) Measurement of pH

The pH of each of the polishing solutions obtained in Examples 1 to 4and each of the polishing agent solutions obtained in Examples 5 and 6was measured at 25° C. by using pH81-11 manufactured by YokokawaElectric Corporation. The measurement results are shown in Table 1.

(3) Evaluation of Polishing Performances

Regarding each of the polishing solutions obtained in Examples 1 to 4and each of the polishing agent solutions obtained in Examples 5 and 6,polishing performances were evaluated by the following methods.

(3-1) Polishing Conditions

Small-sized one side polishing apparatus manufactured by MAT Inc. wasused as a polishing machine. SUBA800-XY-groove (manufactured byNitta-Haas Incorporated) was used as a polishing pad, and conditioningof the polishing pad was conducted by using a diamond disk and a brushbefore polishing.

Polishing was conducted for 30 minutes under the conditions of supplyrate of the polishing solution or polishing agent solution: 25 cm³/min,the number of revolution of a polishing surface plate: 68 rpm, thenumber of revolution of a substrate holding part: 68 rpm, and polishingpressure: 5 psi (34.5 kPa).

(3-2) Material to be Polished

As a material to be polished, 4H-SiC substrate having a diameter of 3inches, having been subjected to a preliminary polishing treatment usingdiamond abrasive was prepared. A single-crystal SiC substrate (On-axissubstrate) in which off angle to C axis of the principal surface (0001)is within 0°+0.25° was used, and Si surface side was polished andevaluated.

(3-3) Measurement of Polishing Rate

Polishing rate was evaluated by amount of thickness change per unit time(nm/hr) of the single-crystal SiC substrate. Specifically, mass of anunpolished substrate having a known thickness and mass of the substrateafter polishing for each period of time were measured, and the masschange was obtained from the difference. The change per unit time of thethickness of the substrate obtained from the mass change was calculatedby using the following equation. The calculation results of thepolishing rate are shown in Table 1.

(Calculation Formula of Polishing Rate (V))

Δm=m0−m1

V=Δm/m0×T0×60/t

(In the formula, Δm (g) is mass change before and after polishing, m0(g) is initial mass of an unpolished substrate, m1 (g) is mass of thesubstrate after polishing, V is polishing rate (nm/hr), T0 is athickness of an unpolished substrate (nm), and t is a polishing time(min))

(3-4) Measurement of Average Line Roughness (R) of Front Edge Portion ofAtomic Step

The principal surface of On-axial substrate after polishing, which hasbeen polished with the respective polishing solutions of Examples 1 to 4and the respective polishing agent solutions of Examples 5 and 6 wasobserved with AFM in an area of 2 μm horizontal×1 μm vertical. As aresult, the formation of an atomic step-and-terrace structure wasconfirmed in all cases. The respective average line roughness (Ra) of aplurality of front edge portions in the above area were measured fromthe AFM images obtained, and its average value was defined as R. D3100(manufactured by Veeco) was used as AFM.

Measurement position of the average line roughness (Ra) of the frontedge portion 2 a is shown by a broken line in FIG. 6. In this Figure,reference numeral 1 indicates a terrace in an atomic step-and-terracestructure.

Proportion (A) of the average line roughness (R) to a height (h) of theatomic step was calculated from the average line roughness (R) of thefront edge portion obtained in (3-4) above by using the followingformula. The result is shown in Table 1.

A(%)=(R(nm)/h(nm))×100

The height (h) of a bilayer atomic step comprising silicon and carbonpair is calculated from 1.008 nm/4 as described above, and is about 0.25nm.

TABLE 1 Proportion of average line roughness Concen- Concen- Polishing(nm) of tration tration rate front edge of Kind of metal of On- portionto Abrasive Secondary Kind of oxidizing of salt axis height of Kind ofconcentration particle size oxidizing agent metal (% by pH substrateatomic step abrasive (% by mass) of abrasive agent (% by mass) saltmass) adjuster pH (nm/hr) (%) Ex. 1 — — — Potassium 1.58 — — Nitric 2523 16 permanganate acid Ex. 2 — — — Potassium 5 — — Nitric 2 962 16permanganate acid Ex. 3 — — — Potassium 3.16 — — Nitric 5 427 20permanganate acid Ex. 4 — — — Potassium 3.16 — — KOH 10 302 16permanganate Ex. 5 Colloidal 20 0.07 Hydrogen 1 Ammonium 0.5 — 6.5 69 24silica peroxide vanadate Ex. 6 Colloidal 20 0.11 Potassium 1.58 — —Nitric 2 55 28 silica permanganate acid

As can be seen from Table 1, in the case of using the polishingsolutions of Examples 1 to 4, high polishing rate is obtained tosingle-crystal SiC On-axis substrate, and high speed polishing waspossible. Furthermore, the formation of the atomic step-and-terracestructure is confirmed from AFM image of the principal surface afterpolishing, and highly-smooth polished principal surface was obtained.Furthermore, in the atomic step-and-terrace structure, the proportion(A) of the average line roughness (R) of the front edge portion of theatomic step to the theoretical value of the height (h) of the atomicstep is 20% or less, and high processing accuracy in atomic level inwhich mechanical damage due to polishing had been suppressed wasobtained. Moreover, because the polishing solution did not contain anabrasive, high cleanliness polished principal surface free of abrasiveresidue was obtained.

On the other hand, in the case of using the polishing agent solution ofExample 5 containing colloidal silica as an abrasive, hydrogen peroxideas an oxidizing agent, and ammonium vanadate, the polishing rate wasremarkably low value as compared with the case of using the polishingsolutions of Examples 1 to 4. Furthermore, although the formation of theatomic step-and-terrace structure is confirmed from AFM image of theprincipal surface after polishing, the proportion of the average lineroughness (R) of the front edge portion of the atomic step to thetheoretical value of the height (h) of the atomic step is 24% which islarger as compared with Examples 1 to 4, and thus, the surface roughnessis deteriorated. It was seen that crack and dent are generated in thefront edge portion of the atomic step by mechanical damage due topolishing. Furthermore, abrasive residue that seems to be colloidalsilica was observed on the polished principal surface.

Even in the case of using the polishing agent solution of Example 6containing potassium permanganate as an oxidizing agent and furthercontaining colloidal silica as an abrasive, the polishing rate wasgreatly decreased as compared with the case of using the polishingsolutions of Examples 1 to 4. Furthermore, although the formation of theatomic step-and-terrace structure was confirmed from AFM image of theprincipal surface after polishing, the proportion of the average lineroughness (R) of the front edge portion of the atomic step to thetheoretical value of the height (h) of the atomic step is 28% which islarger as compared with Examples 1 to 4, and thus, the surface roughnessis deteriorated. It was seen that crack and dent are generated in thefront edge portion of the atomic step by mechanical damage due topolishing. Furthermore, abrasive residue that seems to be colloidalsilica was observed on the polished principal surface.

INDUSTRIAL APPLICABILITY

According to the polishing solution of the present invention, asingle-crystal silicon-carbide substrate having high hardness and highchemical stability can be polished in high polishing rate, and theprincipal surface having high processing accuracy in atomic level inwhich the atomic step-and-terrace structure free of scratch and havingexcellent flatness and smoothness was formed, mechanical damage of thefront edge portion of the atomic step that becomes the origin of crystalgrowth in epitaxial growth in a step flow method was suppressed can beobtained. Therefore, film formation of high quality semiconductor layeron a single-crystal silicon-carbide substrate becomes possible, and thiscan contribute to the improvement in productivity of electronic deviceand the like using a single-crystal silicon-carbide substrate having thesemiconductor layer thus film-formed thereon.

Although the present invention has been described in detail and byreference to the specific embodiments, it is apparent to one skilled inthe art that various modifications or changes can be made withoutdeparting the spirit and scope of the present invention.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

1 . . . Terrace; 2 . . . Atomic step; 2 a . . . Front edge portion; 10 .. . Polishing apparatus; 11 . . . Polishing surface plate; 12 . . .Surface plate drive motor; 13 . . . Polishing pad; 14 . . . Object to bepolished; 15 . . . Substrate holding member; 16 . . . Dropping nozzle;and 17 . . . Polishing solution

1-11. (canceled) 12: A process of manufacturing a single-crystalsilicon-carbide substrate, the process comprising: contacting a surfaceof a single-crystal silicon-carbide plate with a surface of a polishingpad; and moving the surface of the single-crystal silicon-carbide platerelative to the surface of the polishing pad while supplying a polishingsolution to the surface the polishing pad, to polish the surface of thesingle-crystal silicon-carbide plate, wherein the polishing solutioncomprises an oxidizing agent which comprises a transition metal havingoxidation-reduction potential of 0.5 V or more, and wherein neither thepolishing pad nor the polishing solution comprises an abrasive. 13: Theprocess according to claim 12, further comprising holding thesingle-crystal silicon-carbide plate with a carrier, wherein the surfaceof the single-crystal silicon-carbide plate is moved relative to thesurface of the polishing pad by moving the carrier relative to thepolishing pad. 14: The process according to claim 12, wherein a pH ofthe polishing solution is 11 or less. 15: The process according to claim12, wherein a pH of the polishing solution is 5 or less. 16: The processaccording to claim 12, wherein a pH of the polishing solution is 3 orless. 17: The process according to claim 12, wherein the oxidizing agentcomprises a permanganate ion. 18: The process according to claim 12,wherein the oxidizing agent comprises potassium permanganate or sodiumpermanganate. 19: The process according to claim 17, wherein an amountof the permanganate ion in the polishing solution is 0.05 mass % or moreand 5 mass % or less. 20: The process according to claim 17, wherein anamount of the permanganate ion in the polishing solution is 0.1 mass %or more and 4 mass % or less. 21: The process according to claim 17,wherein an amount of the permanganate ion in the polishing solution is0.2 mass % or more and 3.5 mass % or less. 22: The process according toclaim 12, wherein the single-crystal silicon-carbide substrate comprisesa principal surface having an atomic step-and-terrace structure, theatomic step-and-terrace structure comprising atomic steps and terracesderived from a crystal structure, wherein the atomic step-and-terracestructure has a proportion of an average line roughness of a front edgeportion of the atomic step to a height of the atomic step being 20% orless. 23: The process according to claim 11, wherein the principalsurface is a surface on which a crystal is to be epitaxially grown toform a silicon-carbide semiconductor layer or a gallium-nitridesemiconductor layer. 24: The process according to claim 22, wherein awidth of the terrace is substantially the same in each terrace. 25: Theprocess according to claim 22, wherein the front edge portion of theatomic step is a straight line having the average line roughness. 26:The process according to claim 13, wherein a pH of the polishingsolution is 3 or less. 27: The process according to claim 26, whereinthe oxidizing agent comprises a permanganate ion. 28: The processaccording to claim 27, wherein an amount of the permanganate ion in thepolishing solution is 0.1 mass % or more and 4 mass % or less. 29: Theprocess according to claim 27, wherein an amount of the permanganate ionin the polishing solution is 0.2 mass % or more and 3.5 mass % or less.30: The process according to claim 28, wherein the single-crystalsilicon-carbide substrate comprises a principal surface having an atomicstep-and-terrace structure, the atomic step-and-terrace structurecomprising atomic steps and terraces derived from a crystal structure,wherein the atomic step-and-terrace structure has a proportion of anaverage line roughness of a front edge portion of the atomic step to aheight of the atomic step being 20% or less 31: The process according toclaim 29, wherein the single-crystal silicon-carbide substrate comprisesa principal surface having an atomic step-and-terrace structure, theatomic step-and-terrace structure comprising atomic steps and terracesderived from a crystal structure, wherein the atomic step-and-terracestructure has a proportion of an average line roughness of a front edgeportion of the atomic step to a height of the atomic step being 20% orless.